@Nathan Gesner - I think where I'm coming to on this, after all this feedback and discussion, is I should find a middle of the road approach. I think I'm okay showing SOME flexibility with a tenant who has proven themselves to be honest and trustworthy. But also not so flexible that I expose myself to a lot of extra risk. I'm thinking I'll tell them that at a certain day, say day 10 or 14, I will begin eviction. So that gives them some flexibility, but limits my risk as well. That may be a good middle of the road approach...
In terms of the base rent, before late fees, it's actually right around the market average for local comps... maybe slightly on the high end. I have several other properties within a 2 mile radius of this house so I'm well aware of the market. Why they are willing to live like that and pay those late fees, I can't answer. If it were me, I'd move to someplace cheaper that's more appropriate for my budget. I think they really love the house, the area, and me as a landlord. I think they know that most landlords would show no flexibility, so they appreciate that I've been willing to work with them.... as long as they honor the lease, which they have to a T for the last 4 years. But still, I'd prefer a nice easy, low risk tenant who pays on time any day! My other tenants don't have this issue...